Compensation of spacing between magnetron and sputter target

ABSTRACT

A lift mechanism for and a corresponding use of a magnetron in a plasma sputter reactor. A magnetron rotating about the target axis is controllably lifted away from the back of the target to compensate for sputter erosion, thereby maintaining a constant magnetic field and resultant plasma density at the sputtered surface, which is particularly important for stable operation with a small magnetron, for example, one executing circular or planetary motion about the target axis. The lift mechanism can include a lead screw axially fixed to the magnetron support shaft and a lead nut engaged therewith to raise the magnetron as the lead nut is turned. Alternatively, the support shaft is axially fixed to a vertically moving slider. The amount of lift may be controlled according a recipe based on accumulated power applied to the target or by monitoring electrical characteristics of the target.

RELATED APPLICATIONS

This application claims benefit of U.S. provisional application Ser. No.60/529,209, filed Dec. 12, 2003. It also is related to concurrentlyfiled U.S. patent application entitled MECHANISM FOR VARYING THE SPACINGBETWEEN SPUTTER MAGNETRON AND TARGET.

FIELD OF THE INVENTION

The invention relates generally to sputter deposition of materials. Inparticular, the invention relates to a movable magnetron that creates amagnetic field to enhance sputtering.

BACKGROUND ART

Sputtering, alternatively called physical vapor deposition (PVD), is themost prevalent method of depositing layers of metals and relatedmaterials in the fabrication of integrated circuits. The moreconventional type of sputtering, as originally applied to integratedcircuits as well as to other applications, deposits upon a workpiece aplanar layer of the material of the target. However, the emphasis hasrecently changed in the use of sputtering for the fabrication ofintegrated circuits because vertical interconnects through inter-leveldielectrics having the high aspect ratios now being used present a muchgreater challenge than the horizontal interconnects. Furthermore, thehorizontal interconnects are being increasingly implemented byelectrochemically plating copper into horizontally extending trencheswhile sputtering is being reserved for liner layers deposited onto thesidewalls in the holes in which the vertical interconnects are formed oralso deposited onto the walls of the horizontal trenches.

It has long been known that sputtering rates can be increased by the useof a magnetron 10, illustrated in the schematic cross-sectional view ofFIG. 1, positioned in back of a sputtering target 12. The magnetronprojects a magnetic field 14 across the face of the target 12 to trapelectrons and hence increase the plasma density. The magnetron 10typically includes at least two magnets 16, 18 of anti-parallel magneticpolarities perpendicular to the face of the target 12. A magnetic yoke20 supports and magnetically couples the two magnets 16, 18. Theresultant increased plasma density is very effective at increasing thesputtering rate adjacent the parallel components of the magnetic field14. However, as illustrated in the cross-sectional view of FIG. 2, anerosion region 22 develops adjacent the magnetic field, which brings afront surface 24 of the target 12 closer to the magnetron 10, whichfront surface 24 is the surface being currently sputtered. The erosionillustrated in FIG. 2 emphasizes an erosion pit adjacent the magnetron10. In typical operation, the magnetron 10 is scanned over the back ofthe target 12 to produce a more uniform erosion pattern. Nonetheless,even if a target eroded to a planar surface, the fact remains that aftererosion the surface of the target being sputtered is closer to themagnetron 10 than before erosion.

The target erosion presents several problems if the lifetime of thetarget 12 is to be maximized. First, the erosion pattern should be madeas uniform as possible. In conventional planar sputtering, uniformity isimproved by forming the magnets 16, 18 in a balanced, relatively largeclosed kidney-shaped ring and rotating the magnetron about the centralaxis of the target. Secondly, the erosion depth can be compensated byadjusting the spacing between the target and the wafer being sputterdeposited, as disclosed by Tepman in U.S. Pat. No. 5,540,821. Futagawaet al. disclose a variant in U.S. Pat. No. 6,309,525. These schemes haveprimarily addressed the dependence of deposition rate on the separationbetween the wafer and the effective front face of the target 12. Theseapproaches do not address how the erosion affects the magneticenhancement of sputtering.

The erosion problem has been complicated by the need to produce a highlyionized sputter flux so that the ionized sputter atoms can beelectrostatically attracted deep within high aspect-ratio holes and bemagnetically guided, as has been explained for an SIP reactor by Fu etal. in U.S. Pat. No. 6,306,265, incorporated herein by reference in itsentirety. The apparatus described therein uses a small triangularlyshaped magnetron to effect self-ionized sputtering, taking into accountthree factors. First, it is advantageous to reduce the size of themagnetron in order to concentrate the instantaneous sputtering to asmall area of the target, thereby increasing the effective target powerdensity. Secondly, the concentrated magnetic field of the smallmagnetron increases the plasma density adjacent the portion of thetarget being sputtered, thereby increasing the ionization fraction ofthe target atoms being sputtered. The ionized sputter flux is effectiveat being attracted deep within high aspect-ratio holes in the wafer.However, the target erosion affects the effective magnetic field at thetarget face being sputtered, thereby changing the sputtering rate andthe ionization fraction. Thirdly, the small magnetron makes uniformtarget sputtering that much more difficult. Various magnetron shapes,e.g., triangular have been used to increase the uniformity ofsputtering, but their uniformity is not complete. Instead, annulartroughs are eroded into the target even in the case of rotarymagnetrons.

Two major operational effects are readily evident in the use ofconventional rotary magnetrons, particularly small magnetrons. First, asillustrated in the plot 26 of the graph of FIG. 3, the deposition ratefalls from its initial rate with target usage, here measured in targetkilowatt-hours of cumulative power applied to the target since it wasfresh. The target usage corresponds to both the amount of target thathas been eroded since the target was put into service with asubstantially planar and uneroded surface and to the number of wafersthat have been deposited in a repetitive process. We believe that thedecrease is believed arises at least indirectly from the target erosionin which the target surface being sputtered is no longer optimized forthe magnetic field since its separation from the magnetron is varying.The sputtering degradation can be compensated by either increasing thelength or sputtering or increasing the target power. Secondly, thenon-uniformity of sputtering reduces the lifetime of the target to anumber N₁ at which the erosion trough maximum approaches the targetbacking plate or, in the case of an integral target, a minimum thicknessof the target. At this point, to prevent either sputter deposition ofthe backing material or breakthrough of the target, the target must bediscarded even though substantial target material survives away from theerosion troughs. Costs would be saved for target purchase, operatortime, and production throughput if the target lifetime is increased.

Hong et al. have presented a planetary magnetron as a solution to theuniformity problem for a high-density plasma reactor in U.S. patentapplication Ser. No. 10/152,494, filed May 21, 2002, now published asApplication Publication 2003-0217913, and incorporated herein byreference in its entirety. As illustrated in the cross-sectional view ofFIG. 4, a plasma reactor 30 has a fairly conventional lower reactorincluding a reactor wall 32 which supports a sputtering target 34through an adapter 36 and isolator 38 in opposition to a pedestalelectrode 40 supporting the wafer 42 to be sputter deposited with thematerial of the target 34. A vacuum pump system 44 pumps the vacuumchamber to a level of a few milliTorr or less while a gas source 46supplies a working gas such as argon through a mass flow controller 48.A clamp ring 50 holds the wafer 42 to the pedestal electrode 40 althoughan electrostatic chuck may alternatively be used. An electricallygrounded shield 52 protects the reactor walls 32 and further acts as ananode in opposition to the target 34 while a DC power supply 54negatively biases the target 34 to a few hundred volts to excite theargon working gas into a plasma. The positively charged argon ions areaccelerated to the negatively biased target 34, which they strike anddislodge or sputter atoms of the target material. The sputtered atomsare ejected from the target 34 with fairly high energy in a wide beampattern and thereafter strike and stick to the wafer 42. Withsufficiently high target power and high plasma density, a substantialfraction of the sputtered atoms are ionized. Preferably, an RF powersupply 58, for instance oscillating at 13.56 MHz, biases the pedestalelectrode 40 through a capacitive coupling circuit 60 such that anegative DC self-bias develops on the wafer 42, which accelerates thepositively charged sputter ions deep within high-aspect ratio holesbeing sputter coated.

According to the invention, a magnetron 70 positioned in back of thetarget 34 projects its magnetic field in front of the target 34 tocreate a high-density plasma region 72, which greatly increases thesputtering rate of the target 34. If the plasma density is high enough,a substantial fraction of sputtered atoms are ionized, which allowsadditional control over the sputter deposition. Ionization effects areparticularly pronounced in sputtering copper, which has a highself-sputtering yield, as copper ions are attracted back to the coppertarget and sputter further copper. The self-sputtering allows the argonpressure to be reduced, thereby reducing wafer heating by argon ions andreducing argon scattering of copper atoms, whether ionized or neutral,as they travel from the target 34 to the wafer 42.

In the described embodiment, the magnetron 70 is substantially circularand includes an inner magnetic pole 74 of one magnetic polarization withrespect to and extending along a central axis 76 of the chamber 32 aswell as the target 34 and pedestal electrode 40. It further includes anannular outer pole 78 surrounding the inner pole 74 and of the opposedmagnetic polarity along the central axis 76. A magnetic yoke 80magnetically couples the two poles 74, 78 and is supported on a carrier81. The total magnetic intensity of the outer pole 78 is substantiallygreater than that of the inner pole 74, for example by a factor ofgreater than 1.5 or 2.0, to produce an unbalanced magnetron whichprojects its unbalanced magnetic portion towards the wafer 42 to therebyconfine the plasma and also guide sputtered ions towards the wafer 42.Typically, the outer pole 78 is composed of plural cylindrical magnetsarranged in a circle and having a common annular pole piece on the sidefacing the target 34. The inner pole 74 may be composed of one or moremagnets, preferably with a common pole piece. Other forms of magnetronsare encompassed by the invention.

The high plasma densities achieved by this configuration as well as thatof Fu et al. are achieved in part by minimizing the area of themagnetron 70. The encompassing area of the magnetron 70 is typicallyless than 10% of the area of the target 34 being scanned by themagnetron 70. The magnetron/target area ratio may be less than 5% oreven less than 2% if uniform sputtering is otherwise maintained. As aresult, only a small area of the target 34 is subject to an increasedtarget power density and resultant intensive sputtering. That is, thesputtering at any instant of time is highly non-uniform. To compensatefor the non-uniformity, a rotary drive shaft 82 rotated by a drivesource 84 and supporting the magnetron 70 circumferentially scans themagnetron 70 about the chamber axis 76. However, as has been describedwith respect to the reactor of Fu et al., the resultant annular troughsin the target may produce significant radial non-uniformity in thesputtering.

Hong et al. significantly reduce the sputtering non-uniformity by theuse of a planetary scanning mechanism 90 to cause the magnetron 70 tomove along a planetary or other epicyclic path over the back of thetarget 34 with respect to the central axis 76. Their preferred planetarygear mechanism 90 for achieving planetary motion includes, asadditionally and more completely illustrated in FIG. 5, a fixed gear 92fixed to a housing 94 and a drive plate 96 fixed to the rotary shaft 82.In the reactor of Hong et al., the housing 94 is stationary. The driveplate 96 rotatably supports an idler gear 98 which engages the fixedgear 92. The drive plate 96 also rotatably supports a follower gear 100engaged with the idler gear 98. A shaft 102 of the follower gear 100 isfixed also to the carrier 81 so that the magnetron 70 supported on thecarrier 81 away from the follower shaft 102 rotates with the followergear 100 as it rotates about the fixed gear 92 to execute the planetarymotion. Counterweights 110, 112 are fixed to the non-operative ends ofthe drive plate 96 and the carrier 81 to reduce bending and shimmy onthe rotary drive shaft 82 and the follower shaft 102. Particularly incopper sputtering which achieves a high ionization ratio Cu⁺/Cu⁰ ofsputtered copper ions, the sputter reactor 30 of FIG. 4 advantageouslyincludes a magnetic coil or magnet ring 114 annular about the centralaxis 76 to guide the copper ions to the wafer 42.

Because the DC power supply 54 delivers a significant amount of power tothe target 34 and a high flux of energetic ions bombard the target 34thereby heating the target 34, it is conventional to immerse themagnetron 70 as well as the planetary mechanism 90 in a cooling waterbath 116 enclosed in a tank 118 sealed to the target 34 and the fixeddrive-shaft housing 94. Unillustrated fluid lines connect the bath 116with a chiller to recirculate chilled deionized water or other coolingfluid to the bath 118.

The planetary magnetron scanning, because of its convolute path acrossthe target 34, greatly improves the uniformity of target erosion so thatthe target 34 is more uniformly eroded and results in a nearly planarsputtering surface even as the target is eroded. As a result, the targetutilization is greatly improved. Nonetheless, as the target 34 erodesgenerally uniformly, the magnetic field at its sputtering face ischanging and apparently on average decreasing. The change affects thesputtering rate, which as described above has been observed to decrease.The plots presented in FIG. 3 are speculative. Actual experimental dataare presented in FIG. 6. Plot 120 presents the measured deposition ratefor copper in the planetary magnetron chamber of FIG. 4 having anaxially fixed magnetron and with 28 kW of DC target power and 600W of RFbias power as a function of target usage in kilowatt-hours. Plot 121presents the deposition rate for a small axially fixed magnetronexecuting simple rotary motion, as described by Fu et al., with 56 kW ofDC target power. Although the fall off in the simple rotary chamber isnot as great in the planetary chamber, it is still significant. It ispointed out, however, that it may be advantageous to more heavilysputter the outer regions of the target 34, particularly when thetarget/wafer spacing is relatively small in order to compensate for thegeometric effect of greater deposition at the wafer center. Suchintended non-uniformity can be achieved by adjusting the length of therotation arms in a planetary chamber or by changing the shape or radialposition of the magnetron in a simple rotary chamber. Even in this case,the deposition rate decreases with target usage.

A second set of non-uniformity problems is not immediately addressed bythe planetary scanning mechanism. The small area of the magnetron 70advantageously produces a high target power density and high plasmadensity and hence increases sputtering rate and increases the fractionof ionized sputter atoms which are drawn deep within high aspect-ratioholes to coat the sides and bottom of via holes. However, the magneticfield and hence the plasma density depend upon the distance between thetarget sputtering surface and the magnetron. As a result, as the target34 is being sputtered, even if uniformly, the plasma density is changingand hence the sputtering rate and the ionization rate upon which the viasidewall coverage depends are changing. The effect is exacerbated for asmall magnetron because the gradient of the magnet field is greater. Asa result, the changing magnetic field and plasma density destabilizesthe process causing variation in bottom and sidewall coverages acrossthe lifetime of the target. It has generally been accepted that thehigh-performance sputtering is different at the end of the lifetime ofthe target than at the beginning. Plot 122 in FIG. 7 shows the measuredtarget voltage and plot 123 in FIG. 8 shows the measured mean biasvoltage with respect to target usage for the axially fixed planetarymagnetron with the aforementioned values of target and bias power. Thereis a significant rise in the target voltage and the magnitude of thebias voltage with increased sputtering. However, the bias voltage issubject to fluctuations of about +20V with the maximum magnitude greatlyincreasing to about 150V at maximum usage. The instability is readilyapparent from the plot 122 of FIG. 7 for target voltage and the plot 123of FIG. 8 for bias power. The change of sputtering rate can becompensated by increasing the sputtering duration, but this does notaddress the sidewall coverage. In any case, the increased sputteringperiod decreases throughput and introduces another variable into thequeuing plan. The variation in plasma density because of reducedmagnetic field can be partially compensated by increasing the targetpower. Such power compensation however involves an ad hoc relationshipwhich needs to be determined for each set of conditions and also reducesthe ability to maximize plasma densities and sputtering rates withlimited power supplies.

Halsey et al. in U.S. Pat. No. 5,855,744 show an apparatus for deforminga linear magnetron as it scans across a rectangular target. In oneembodiment, multiple actuators moving shafts along multiple respectiveaxes deform the magnetron. Mizouchi et al. in U.S. Pat. No. 6,461,485discloses a single vertical actuator for compensating for end effects inlinear scanning.

Demaray et al. in U.S. Pat. No. 5,252,194 discloses a slider mechanismfor vertically moving a large magnetron to adjust the magnetic field atthe front of the target.

Schultheiss et al. in U.S. Pat. No. 4,927,513 discloses a magnetron liftmechanism to control magnetic properties of sputtered layers.

SUMMARY OF THE INVENTION

The invention includes the method and apparatus for compensating erosionof a plasma sputtering target by moving the magnetron away from the backof the target as the front of the target is eroded. The compensationprovides a more constant magnetic field and plasma density at thesurface of the target being sputtered and results in a more stablesputtering process.

The lift mechanism may include a lead screw mechanism including a leadscrew and lead nut. The lead screw may be axially fixed to the magnetronand a lead nut threadably engaged with the lead screw. Rotation of thelead nut vertically moves the magnetron. The lead screw may beazimuthally fixed while the lead nut is axially fixed. The lead nut maybe manually moved or moved under the control of a motor or otheractuator coupled to the lead nut by a gear or a linear lead screwmechanism or linear actuator.

The amount of lift my be dictated by a predetermined recipe or by ameasured cumulative power applied to the target. Alternatively, thetarget resistance or power characteristic or the physical erosion depthmay be monitored to determine when additional lift is required.

The magnetron lift mechanism may also be used to control the magneticfield at the face of the sputtering target for control of the sputteringprocess other than simple compensation of target erosion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional views functionally illustrating theeffect of magnetron sputtering as the target is being eroded.

FIG. 3 is a graph illustrating the dependencies in the prior art andaccording to the invention of the sputtering deposition rate as afunction of the target usage.

FIG. 4 is a schematic cross-sectional view of a plasma sputter reactorwith a planetary magnetron.

FIG. 5 is an isometric view of a planetary magnetron.

FIG. 6 is a graph illustrating the experimentally determineddependencies in the prior art of the sputtering deposition rate as afunction of target usage.

FIG. 7 is a graph illustrating the experimentally determineddependencies in the prior art and in the practice of the invention oftarget voltage as a function of target usage.

FIG. 8 is a graph illustrating the experimentally determineddependencies in the prior art and in the practice of the invention ofbias voltage as a function of target usage.

FIG. 9 is a graph illustrating as a function of target usage both asequence of target spacings used in an experiment verifying theinvention and the resultant sputtering deposition rate.

FIG. 10 is a cross-sectional view of a lead screw lift mechanism forraising the magnetron.

FIG. 11 is an orthographic view of the exterior portions of the leadscrew lift mechanism of FIG. 10 and a spur gear mechanism for drivingthe spacing between target and magnetron.

FIG. 12 is an orthographic view of a first embodiment of a lead screwmechanism for driving the spacing compensation.

FIG. 13 is an orthographic view of a dual slider mechanism for drivingthe spacing compensation to be used in the lead screw mechanism of FIG.10.

FIG. 14 is an orthographic view of a collar, slider and its case to beused in the slider mechanism of FIG. 13.

FIG. 15 is an orthographic view of a bracket used with the slidermechanism of FIG. 13.

FIG. 16 is an orthographic view of the slider mechanism of FIG. 13additionally including a magnetron rotation motor.

FIG. 17 is a schematic representation of a computerized lift controlsystem.

FIGS. 18 and 19 are schematic cross-sectional view of two hollow cathodemagnetrons incorporated the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The erosion of the front of the target in magnetron sputtering can becompensated by moving the magnetron away from the back of the target. Asillustrated in FIG. 4, a lift mechanism 124 controllably raises themagnetron 70 with respect to the back of the target 34, preferably in anamount commensurate to the amount of the front surface of the target 34that has been eroded since the target 34 was installed with a fleshplanar front surface. The compensation should focus on the areas of thetarget 34 being more heavily eroded since they contribute a higherfraction of sputtered atoms. While the conventional design criterionminimizes the distance between the magnetron 70 and the back of thetarget 34 and maintains the separation at this initial spacing, onepreferred criterion of the invention maintains an approximately constantspacing between the magnetron 70 and the front of the target 34 facingthe wafer 42. The nearly constant spacing maintains a substantiallyconstant magnetic field at the surface of the target 34 being sputtered.The nearly constant magnetic field removes one variable from the processconditions determining sputtering performance, not just sputtering ratebut also ionization fraction among other effects. Thereby, sputteringtime or target voltage do not need to be adjusted for target usage.Movement of the magnetron to maintain a substantially constant magneticfield at the front face of the target 34 stabilizes the sputteringprocess over the life of the target and enables a substantially constantdeposition rate despite target usage, as schematically illustrated inplot 126 of FIG. 3. Also, the sidewall and bottom coverages may bemaintained substantially constant over the life of the target.Furthermore, the lifetime of the target considerably increases to avalue N₂ as the target is nearly uniformly eroded almost to its backingplate.

Although other implementations are possible, the lift mechanism 124 canbe easily incorporated into the conventional design by allowing thehousing 94 to be axially movable by the lift mechanism while stillmaintaining its fluid seal to the tank 118.

A first embodiment of the invention used to verify the effects ofcompensating the magnetron-to-target spacing uses a series of shims ofvarying thickness placed between the magnetron 70 of FIG. 5 and thecarrier 81. As the target erodes, the previous shim is replaced by athinner shim. As a result, the magnetron 70 is being moved away from theback of the target 34 along a single axis at the center of the magnetron70 although that axis is moving as the magnetron 70 is moved along aplanetary path. As a result, a nearly constant magnetic field can bemaintained at the sputtering surface of the target 34 over the targetlife thereby stabilizing the sputtering process. The manual shimmingprocess may be alternatively effected by shims placed between theotherwise stationary housing 94 and the roof of the tank 118.

Actual experimental data using a copper target and a planetary magnetronin the reactor of FIG. 4 are presented in FIG. 9. Plot 125 shows themagnet-to-target (MTS) spacing, specifically to the back of the targetas the shim thickness was occasionally increased while plot 126 in showsthe measured deposition rate. Even in these preliminary experiments, thedeposition rate is maintained nearly constant. With the use of theinvention, a copper target may be used to deposit thin copper seedlayers on up to 20,000 wafers. Plot 121 in FIG. 6 also shows actual datafor the moderated change of deposition rate as a function of targetusage as the shims were being replaced. Plot 127 in FIG. 7 shows thedependence of measured target voltage as a function of target usage.Plot 128 in FIG. 8 shows the dependence of the mean of the measured biasvoltage as a function of target usage. Further, although notillustrated, the deviations of the maximum and mean bias voltage fromthe mean do not significantly change with target usage.

These results could be improved particularly for target and bias powerby more frequently moving the magnetron with a finer resolution. Theseresults also show that target and bias voltages are sensitive indicatorsof the amount of erosion and hence the need for spacing compensation.These voltages are easy to monitor during production. Current is anothersensitive measurement for electrical supplies generating constant power.Alternatively, if the power supplies are set to generate constantvoltage or current, the complementary quantity or power may be measured.These electrical measurements typically amount to monitoring theresistance of the plasma under some set electrical condition. Therefore,the compensation can be dynamically controlled by measuring one or bothof these voltages (or other quantities) during production and comparingthem to baseline values. When the deviation exceeds a threshold, thecompensation may be performed to bring the measured value closer to thebaseline value. It is also possible to optically or otherwise measurethe physical depth of erosion of the target and use the depthmeasurement to initiate the compensation. Nonetheless, it has provensatisfactory to keep track of cumulative target power and move themagnetron at values experimentally determined for a given sputterrecipe.

Although the first embodiment relying on shims is effective, it clearlypresents operational difficulties as the sputter reactor needs to beshut down and the magnetron removed from the water bath to allow manualreplacement of its shims. It is greatly desired to perform the spacingcompensation from outside the water bath and preferably undercomputerized electrical control.

One set of embodiments is based on converting the stationary housing 94to a vertically movable but in large part azimuthally fixed housing 94driven by a lead screw mechanism 130, as illustrated in thecross-sectional view of FIG. 10. The rotary drive shaft 82 includes acentral bore 132 for flowing chilled cooling water to the center of thetank 118 of FIG. 4 near the back of the target 34. The cooling waterflows out of the water bath through an unillustrated outlet in a roof142 or other wall of the tank 118. The top of the drive shaft 82 iscoupled by yet further unillustrated belts or other means to the motor84. The drive plate 96 of the planetary mechanism is fixed to the bottomof the drive shaft 82 and rotates with it. Two ring bearings 134, 136rotatably support a boss 138 of the drive shaft 82 within the housing94. An annular dynamic seal 139 the seals the fluid within the bath 116from the bearings 134, 136 and the exterior.

A tail 140 of the housing 94 axially passes through an aperture in thetank roof 142 but is azimuthally fixed by other means. The fixed gear 92of the planetary mechanism is fixed to the end of the housing tail 140.As a result, when the housing 94 is vertically moved, the fixed gear 92,the drive plate 96, and the rest of the planetary mechanism 90 andmagnetron 70 are also vertically moved along the central axis 76.

A support collar 146 is fixed to the tank roof 142 and sealed to it withan O-ring placed in an O-ring groove 147. An annular bellows 148surrounding the upper portion of the housing tail 140 is sealed onopposed ends to the housing 94 and to the inner portion of the supportcollar 146 to slidably seal the fluid in the bath 116 from the exterioras well as from most of the mechanical parts of the lift mechanism 130while allowing axial movement between the housing 94 and drive shaft 82on one hand and the tank roof 142 on the other. The bellows 148 shouldaccommodate a movement of about ¾″ (2 cm) corresponding to the usablethickness of the target 34. Other types of slidable fluid seals arepossible. The fixed collar 146 rotatably supports an internally threadedlead nut 150 through two ring bearings 152, 154. An inner retainer ring156 fixed to the lead nut 150 and an outer retainer ring 158 fixed tothe collar 146 trap the upper bearing 152 against the lead nut 150 andthe collar 146. Another similar retainer ring configuration beneath thelead nut 150 traps the lower bearing 154. The lead nut 150 can thusrotate about the central axis 76 but is axially fixed to the tank top142.

The external threads of a azimuthally fixed but vertically movable leadscrew 164 engage the internal threads of the lead nut 150. The leadscrew 164 supports the housing 94 on its upper surface. The housing 94may be fixed to the lead screw 164 or guide pins may couple them toprevent relative rotational movement. A plurality of screws 166 hold thelead screw 164 to the tank top 142 through compression springs 168. As aresult, the lead screw 164 is rotationally fixed as it engages therotatable lead nut 150 but the compression springs 168 accommodatelimited vertical motion of the lead screw 164. The axial fixing of thelead nut 150 to the tank top 142 provides a wide mechanical base for theheavy rotating magnetron, thereby reducing shimmy and allowing thereduction of the clearance between the magnetron 70 and the back of thetarget 34.

In operation, if the lead nut 150 is rotated clockwise, the azimuthallyfixed lead screw 164 rises and lifts the housing 94 and the attachedrotary shaft 82 and magnetron 70 away from the target 34.Counter-clockwise rotation of the lead nut 150 produces the oppositeaxial movement of lowering the magnetron 70 toward the target 34. Thelift drive mechanism for rotating the lead nut 150 is easily formedoutside of the cooling bath 116. Two types of lift drive mechanisms willbe described.

A first embodiment of a rotational lift drive includes a spur gear drive170 illustrated in the orthographic view of FIG. 11. The outer rim ofthe inner retainer ring 156 is partially formed with a toothed gear 172in a gear ledge 173 extending from only part of the inner retainer ring156. The toothed gear 172 engages with a lift drive gear 174controllably driven by a lift motor 176, which maybe mounted on the tankroof 142 with a vertically oriented drive shaft to which the toothedgear 172 is fixed. The lift motor 176 is preferably a stepper motorrotating a fixed angle for each motor pulse with a separate controlsignal controlling the direction of rotation. Thereby, the lead nut 150of FIG. 10 is controllably rotated to raise or lower the lead screw 164and hence the housing 94 and attached drive shaft 82 and magnetron 70.

An optical position sensor 175 includes two arms 175 a, 175 b spaced toaccommodate the gear ledge 173 as it rotates in lifting the magnetron.One arm 175 a contains an optical emitter, such as an light emittingdiode, while the other arm 175 b contains a light detector, such as aphotodiode. The position sensor 175 is used to calibrate the rotation ofthe gear 172 using the gear ledge 173 as a flag. The lift motor 176rotates the gear 172 toward the position sensor 175 until the gear ledge173 enters between the arms 175 a, 175 b of the position sensor 175 andblocks the emitted light from the optical detector. The controller notesthat position as a home position. The stepper motor 176 is then steppedin the opposite direction by a controlled number of pulses to a desiredrotation location of the gear 172 and hence vertical position of themagnetron. Other position sensors may be used.

The drive shaft motor 84 may be vertically mounted on the tank roof 142through a motor mount 180. The drive shaft motor 84 drives a motor drivegear 182 through optional unillustrated gearing to reduce the rotationrate. A shaft drive gear 186 is formed in a capstan 188 fixed to thedrive shaft 82. A ribbed belt 190 is wrapped over both the motor drivegear 182 and the shaft drive gear 186 so that the motor 84 rotates thedrive shaft 82 in executing the planetary motion of the magnetron 70.Because the drive shaft 82 and the attached shaft drive gear 186 areraised and lowered in operation relative to the motor mount 180 andattached motor drive gear 182, the teeth of at least the shaft drivegear 186 must be wide enough to accommodate the slip or axial movementof the belt 190 relative to teeth of that gear 186 and the motor drivegear 182 may be formed with two rims to limit the axial movement of thebelt 190 on that gear 182. A rotary fluid coupling 194 is mounted on thetop of the drive shaft 82 to allow cooling water lines to be connectedto the central bore 132 of the rotating drive shaft 82.

A second embodiment of a rotational lift drive includes a lead screwmechanism 200 illustrated in the orthographic, partially sectioned viewof FIG. 12. A support collar 202 is fixed to the tank roof 142 androtatably supports the lead nut 150 through a ring bearing 204 trappedby an upper retainer ring 206. A lead nut lever 208 extends radiallyoutwardly from the lead nut 150 and has two parallel arms 210 formed atits end. A pivot connection including two arms 212 at the back of thelift motor 176, an unillustrated pivot pin through them, and a mount forthe pin, pivotally mounts the lift motor 176 to the tank roof 142 in ahorizontal orientation. The horizontally oriented lift motor 176 rotatesa shaft 216 having a lead screw formed on its distal end. A nut box 220threadably captures the lead screw of the drive shaft 216 and ispivotally supported by a pin 218 fixed to the arms 210 of the lead nutlever 208. Thereby, rotation of the lift motor 176 rotates the lead nut150 to raise or lower along the central axis 72 the lead screw 164 andhence the housing tail 140 and attached drive shaft 82 and magnetron 70.

The second embodiment of FIG. 12 can be easily modified to replace thelift motor 176 with a hydraulic or pneumatic linear actuator driving ashaft pivotally coupled at its end to the arms 210 of the lead nut lever208. Yet further, the second embodiment could be manually controlled bythe operator manually rotating the lead nut lever 208. Othercombinations of gears, levers, and actuators or motors can be used toimplement the lead nut lift mechanism.

The lead nut lift mechanism offers several advantages. It is concentricabout the lift axis and the support shaft for the magnetron. Themagnetron is supported on an azimuthally fixed lead screw threaded intoa larger lead nut that is axially fixed to a yet larger structure.Hence, the lead nut lift mechanism offers low vibration and flexing ofthe relatively heavy rotating magnetron. The design is mechanicallysimple, thereby increasing reliability and reducing cost.

A second type of lift mechanism is a double slider mechanism 230illustrated orthographically in FIG. 13. It includes an elongated collar232, also illustrated orthographically in FIG. 14, adapted to be fixedand sealed to the tank roof 142. A vertically oriented slider case 234fixed to the collar 232 includes two vertically extending andhorizontally stacked tracks, one of which is formed on one side by rails235. The two tracks respectively trap two sliders 236, which are fixedtogether and can together vertically move along the rails 235. Only theexterior slider 236 is illustrated. Two sliders fixed to each otherprovide greater stiffness in supporting the relatively heavy load. Avertically extending back 238 is fixed to the collar 232 to provide arigid mount for the slider case 234 and other parts. A verticallyoriented drive shaft 240 is rotatably supported in the top end of theslider case 234. The lower end of the drive shaft 240 is threaded as alead screw and engages corresponding threads formed in an unillustratedlead box axially supporting both sliders 236. As a result, when thedrive shaft 240 rotates, the sliders 236 move up or down within theslider case 234.

A bracket 250 illustrated orthographically in FIG. 15 has a base 252sized to fit onto the exterior slider 234. A plurality of through holes254 drilled through the bracket base 252 pass screws 256 threaded intothe exterior slider 234 to snugly hold the bracket base 252. Locatingpins 258 may be inserted into the exterior slider 234 to engagecorresponding holes formed in the bottom of the bracket base 252. Thebracket 250 further includes a tubular collar 260 having an aperture 262sized to closely hold the housing 94 of FIG. 10, with suitablemodifications of that housing 94, and an upper axial end 264 to supportthe housing 94. The housing 94 may be fixed to the tubular collar 260 orengage it through pins so that the housing 94 does not rotate.

Returning to FIG. 13, a shaft gear 270 is fixed to the magnetron driveshaft 82 of FIG. 10, which is rotatable within but vertically fixed inthe housing 94. The housing 94 itself is not rotatable but can movevertically with respect to the collar 232 and the tank roof 142. Thevertically movable housing 94 maybe sealed to the tank roof 142 by anassembly including the bellows 148 of FIG. 10 to be axially movable withrespect to the tank roof 142 over a limited throw. The shaft gear 270 issimilar to the shaft gear 186 of FIG. 11 and may be driven by the belt190 driven by the vertically oriented motor 84. The belt 190 as appliedto FIG. 13 should be able to vertically slide along the teeth of theshaft gear 270.

A slider drive mechanism includes a plate 276 fixed to end of the slidercase 234 which passes the end of the slider shaft 240 to be fixed to aslider gear 278. The plate 276 also supports below a vertically orientedslider motor 280 having a drive shaft fixed to a motor gear 282. Aribbed belt 284 is wrapped around the slider and motor gears 278, 282 sothat the slider motor 280 can move the slider 234 up and down within theslider case 250 to thereby vertically move the housing 94 and attachedmagnetron 70 relative to the tank roof 142 and the back of the target34.

A modified double slider mechanism 290 illustrated orthographically inFIG. 16 includes a modified bracket 292 having a shelf 294 extendingoutward from the top of the tubular collar 260. A shaft drive motor 296and gear box 298 are supported on the bottom of the shelf 294 to drive amotor gear 300. A ribbed belt 302 engages both the motor gear 300 andthe shaft gear 270 to thereby rotate the magnetron shaft 82 and causethe attached magnetron 70 to execute planetary motion. Because the motor296 and motor gear 300 are axially fixed to the housing 94 and hencemoves with the magnetron shaft 82, the belt 270 does not need to slipalong the teeth of the gears 270, 300.

The described compensation mechanisms may be used in a number of waysfor compensating target erosion. It is possible to perform the liftingand compensating during the plasma excitation and sputter deposition,but it is preferable instead to perform it after one wafer is processedand before the next one is processed. Even though motor controlled, themechanisms may be essentially manually controlled by on occasioninstructing the lift motor to move a set amount corresponding to adesired lift of the magnetron. However, the lift compensation algorithmis advantageously incorporated into the recipe for which a machine isbeing used and a computerized controller performs the compensation aswell as controls the other chamber elements according to the recipe. Inview of the limited axial throw of about 2 cm and the large number ofwafers which maybe deposited with a single target over many weeks ofeven continuous processing, it is reasonable to compensate the spacingon only an occasional basis, for example, once an hour or once a day ormore specifically after a large number of wafers have been processed.

In a control procedure emphasizing the optimized process in which thereactor is being used, the amount of displacement may be determinedempirically for a given combination of target, magnetron, initialtarget/magnetron spacing, and general operating conditions developed fora step in the fabrication of a chip design A convenient unit of targetusage is total kilowatt-hours of use since the target was fresh so thatthe process recipe keeps a running total of kilo-watt hours and adjuststhe spacing as a function of the total kilowatt-hours according to acompensation algorithm incorporated into the process recipe and setduring development of the recipe. The compensation may be controlledonce a set period for this unit has passed. For a given process, wafercount is nearly as good a usage unit.

A dynamic control algorithm may also be effective. As is evident fromthe plots 122, 127 of FIG. 7, the measured target voltage can be trackedand correlated with deviations from a predetermined value set by therecipe. when the measured voltage deviates by a set voltage increment,the magnetron maybe moved upwardly by a set spacing incrementexperimentally determined beforehand to largely compensate the voltageincrement. In fact, the empirical algorithm may be obtained incorresponding fashion during development of the process in which thedevelopment engineer tracks changes in the target voltage as a functionof target usage and experimentally determines what spacing compensationis necessary to bring the target voltage back to a set value.

It is also possible to directly measure the position of the sputteringsurface of the target by optical or other means or to measure thethickness of the target by separate electrical means, both approachesproviding a measurement of target erosion.

It is desirable that the compensation be directly measurable by afeedback measurement, for example, the angular position of the set nutor of the linear position of the slider or an angular displacement ofone of the rotary parts, all measured from a known position. Forexample, the position sensor 175 of FIG. 11 acts as a limit indicatoruseful for resetting after a power outage or computer glitch.

It is noted that the baseline magnetron-to-target spacing may vary fromone recipe to another and the described lift mechanisms may be used toinitially obtain the baseline spacing for a fresh target as well as tomaintain it during extended target usage.

FIG. 17 schematically illustrates an example of a control system foradjusting the spacing S between the front face of the magnetron 70 andthe back surface of the target 34 as well as controlling other parts ofthe sputter reactor. The lift motor 176 is preferably implemented in astepper motor that is connected through a schematically illustratedmechanical drive 310 (for example that of FIGS. 10 and 11) which canselectively raise or lower the magnetron 70. A flag 312 attached to themechanical drive 310, and a position sensor 314 detects the position ofthe flag 312, for example, at the extreme of the travel of themechanical drive 310 in which the magnetron 70 is farthest from thetarget 34.

A computerized controller 316 is conventionally used to control thesputtering operation according to a process recipe stored within thecontroller 316 on a recordable medium 318, such as a recordable disk.The controller 316 conventionally controls the target power supply 54 aswell as other conventional reactor elements 44, 48, 58, 84, and 114.Additionally according to the invention, the controller 316 controls thestepper motor 176 with a controlled series of pulses and a directionalsignal to drive the magnetron 70 a controlled distance in eitherdirection. The controller 316 stores the current position of themagnetron 70 and, if additional movement is desired, can incrementallymove the magnetron 70. However, on startup or after some unforseeninterrupt, the controller 316 raises the magnetron 70 away from thetarget 34 until the position sensor 314 detects the flag 312. Thesetting of the stepper motor 176 at this flagged position determines ahome position. Thereafter, the controller 316 lowers the magnetron 70 toa desired position or spacing S from the target 34. This limit detectionmaybe implemented by the position sensor 175 of FIG. 11.

The recipe stored within the controller 316 may contain the desiredcompensation rate, for example, as a function of kilowatt hours of powerapplied to the target 34 from the power supply 54 or alternatively as acompensation for variation in target voltage. The controller 316 canmonitor the applied power through a watt meter 320 connected between thepower supply 54 and the target. However, the power supply 54 is oftendesigned to deliver a selectable constant amount of power. In this case,the total power consumption can be monitored by software within thecontroller 316 with no direct power measurement. The controller 316 mayalso monitor the target voltage with a voltmeter 322 connected to thepower supply line to the target 34. As mentioned previously, targetvoltage is a sensitive indicator of the need to compensate the spacingbetween magnetron and target.

The spacing compensation may be advantageously applied to the roofmagnetron used with a target having an annular vault formed in itssurface, as has been described by Gopalraja et al. in U.S. Pat. No.6,451,177, incorporated herein by reference in its entirety. Theinvention can also be applied to a sputter reactor having a hollowcathode magnetron 330 schematically illustrated in FIG. 18, such asdisclosed by Lai et al. in U.S. Pat. No. 6,193,854, incorporated hereinby reference in its entirety. The hollow cathode magnetron 330 includesa target 332 formed with a single right circular cylindrical vaultextending about a central axis 334 and facing an unillustrated pedestalsupporting the wafer. Unillustrated biasing means applied to the target332 relative to an anode excites the sputtering working gas into aplasma to sputter the portions of the target 332 inside the vault tothereby coat a layer onto the wafer of the material of the target 332.

Permanent magnets 336, usually axially aligned, are placed around theexterior of a circumferential sidewall 338 of the target 332 to serveseveral functions including intensifying the plasma adjacent thesidewall 338. However, in some implementations, the magnets arehorizontally aligned to create a bucking field within the vault adjacentthe sidewall 338. According to the invention, motors or other types ofactuators 340 selectively move the magnets 336 radially with respect tothe central axis 334 to compensate for sputtering erosion of the targetsidewalls 338. The hollow cathode magnetron 330 may additionally includea roof magnetron 342 positioned in back of a disk-shaped roof 342 of thetarget 332. The roof magnetron 342 may be stationary or be rotated aboutthe central axis 334. According to the invention, a motor or otheractuator 346 maybe used to axially move the roof magnetron 342 along thecentral axis 334 to compensate for erosion of the target roof 344.However, as has been previously discussed, the various magnet movementsmay be used alternatively to tune the sputtering process to an initialstate as well as to maintain it there.

An alternative hollow cathode magnetron 350 schematically illustrated inFIG. 19 uses a sidewall coil 352 wrapped around the target sidewall 338to produce an axial magnetic field inside the target vault. According tothe invention, an adjustable power supply 354 supplying the coil currentis adjusted, for among other reasons, to compensate for target erosionsuch that a more constant magnetic field is produced adjacent theinterior surface of the eroding target.

The compensation mechanism is not limited to those which have beendescribed. For example, especially in the case that the magnetronexecutes only simple rotary motion, the rotary shaft supporting themagnetron can be directly lifted if an additional dynamic or slidableseal allows leak-free axial movement of the rotary shaft. Other types oflift mechanisms and lift drives may be used in achieving the control orcompensation of the target/magnetron spacing However, the lead-screwlift mechanism 130 of FIG. 10 has effectively been used for compensatingan SIP magnetron of the Fu patent which executes simple rotary motion.

Although the above described lift mechanisms have been described forraising a magnetron away from the target backside, they may be used aswell to lower the magnetron. Also, the apparatus maybe used for purposesother than compensating for target erosion.

Although the invention has been developed for copper sputtering, it maybe used for sputtering other materials dependent on the target materialand whether a reactive gas is admitted to the chamber. Such materialsinclude nearly all metals and metal alloys and their reactive compoundsused in sputter deposition, including but not limited to Cu, Ta, Al, Ti,W, Co, Ni, NiV, TiN, WN, TaN, Al—Cu alloys, Cu—Al, Cu—Mg, etc.

The invention may be also applied to other magnetrons such as the moreconventional large kidney-shaped magnetrons and to other magnetrons notintended to ionize the sputtered atoms. Nested magnetrons are notrequired. Long-throw sputter reactors can benefit from the invention.Inductive RF power may be coupled into the magnetron sputter reactor toincrease the source power. Although the invention is particularly usefulwith scanned magnetrons, it may also be applied to stationarymagnetrons. It may also be applied to magnets used more for confiningthe plasma and guiding ions rather than strictly for increasing theplasma density.

Accordingly, the invention greatly stabilizes a sputtering process overthe lifetime of the target with relatively minor additions to thesputter apparatus.

The above described embodiments do not encompass all possibleimplementations and uses of the invention. The coverage of the inventionshould be determined primarily by the specific language of the claims.

1. In a magnetron sputter reactor including a chamber sealable to asputtering target, a support in said reactor for holding a substrate tobe processed, and a magnetron placeable on a backside of said targetopposite said support, a magnetron control system comprising: a liftmechanism affixed to a support of said magnetron capable of varying adistance of said magnetron from said backside of said target; and acontroller for controlling a degree of said varying said distance duringprocessing of a sequence of substrates.
 2. The lift mechanism of claim1, including a motor driving said lift mechanism in response to saidcontroller.
 3. A sputter reactor, comprising: a target affixed to vacuumchamber; a pedestal within said chamber for support a substrate inopposition to said target; a magnetron positioned on a back side of saidtarget; a power supply selectively applying power to said target toexcite a plasma within said chamber to thereby sputter material from afront side of said target onto said substrate; a mechanism for varying aspacing between said magnetron and said target; and a controllercontrolling said mechanism to adjust said spacing during a predeterminedsequence of depositing said material onto a plurality of saidsubstrates.
 4. The reactor of claim 3, wherein said controller adjustssaid spacing to compensate for an erosion of said front side of saidtarget by said plasma.
 5. A method of sputtering onto a substratesupport on a support in a sputtering reactor chamber including a targetfixed to said chamber and a magnetron disposed on a back of said targetopposite said support, comprising: plural first steps of exciting aplasma in said chamber and depositing material of said target ontosequential ones of a plurality of substrates; and plural second steps oflifting said magnetron away from said backside of said target performedduring or after different ones of said first steps.
 6. The method ofclaim 5, further comprising rotating said magnetron about a central axisof said target.
 7. The method of claim 5, wherein said rotating stepcauses said magnetron to execute planetary motion about said centralaxis.
 8. The method of any claim 5, wherein said second steps lift saidmagnetron to compensate for an erosion depth of said target after saidtarget enters service.
 9. A magnetron sputtering method, comprisingadjusting a magnetic field, which is assisting generation of a plasma insputtering a target, in compensation for erosion of a front of saidtarget by particles from said plasma.
 10. The method of claim 9, whereinsaid adjusting comprises moving a magnetron away from a back of saidtarget.
 11. The method of claim 9, wherein said adjusting reducesvariations in said magnetic field at said front of said target caused bysaid erosion.
 12. The method of claim 9, wherein said adjusting reducesvariations in said plasma at said front of said target caused by saiderosion.
 13. The method of claim 9, wherein a degree of said adjustingis determined according to a recipe for said sputtering.
 14. A methodfor use with a plasma sputter reactor chamber mounting a target facingan interior of the chamber and with a magnetron positioned on thebackside of said target exteriorly of the chamber, comprising the stepsof: exciting a plasma in said chamber so as to deposit material of saidtarget onto sequential ones of a plurality of substrates; lifting saidmagnetron away from the backside during or after said exciting of saidplasma or deposition of material; and a controller controlling a degreeof said lifting at an intermediate time of processing of said pluralityof substrates.
 15. The method of claim 14, wherein said controllercontrols said degree of said lifting according to an amount of erosionof a front side of said target during processing of said plurality ofsubstrates.
 16. The method of claim 14, wherein there is no substantialeffective movement of said magnetron towards said target during asequence of depositions of said material onto different substrates. 17.The method of claim 16, in which said lifting of said magnetron isperformed during or after any one or more of said exciting of saidplasma or said depositing of material.
 18. The method of claim 16, inwhich said lifting is at performed plural different times.
 19. Themethod of claims 14, further comprising rotating said magnetron about acentral axis of said target to execute circular movement about saidcentral axis.
 20. The method of claim 14, further comprising rotatingsaid magnetron to execute planetary motion about said central axis. 21.A hollow cathode sputter reactor, comprising: a vacuum chamber arrangedabout a central axis; a support within said chamber for holding asubstrate to be processed; a sputter target sealable to said chamber andhaving a right cylindrical vault arranged about said central axis inopposition to said support formed by a generally disk-shaped roof and agenerally tubular sidewall; a magnet assembly disposed adjacent a backsurface of either said roof or said sidewall; and a lift mechanismsupporting said magnet assembly and controllably varying a distancebetween said magnet assembly and said back surface.
 22. The reactor ofclaim 21, wherein said lift mechanism includes an actuator controllablymoving a support member of said magnet assembly.